The Striped Pajama Squid blends into a seabed substrate of broken shells. Coating scientists in California are taking note and trying to emulate the effect.

By understanding how octopi and other marine animals change color to blend into their environment, a team of researchers in California is working to convert the natural processes of the creatures into an artificial camouflage coating.

At the Henry Samueli School of Engineering at the University of California, Irvine, Assistant Professor Alon A. Gorodetsky's laboratory is working on using standard infrared detection equipment to make things appear and disappear.

The secret ingredient: squid protein.

Color-Changing Capabilities

Cephalopods, such as squids and octopuses, have skin cells that can reflect visible and infrared light. These cells contain unique proteins called reflectins that, when set off by acetylcholine, a neurotransmitter, allow a set of proteins to condense, setting off the color-changing process.

Researchers at the University of California, Irvine, are developing a color-changing coating that would react to one's surroundings in the same manner as an octopus or squid.

"We're trying to develop something that you could essentially use as reconfigurable infrared reflective paint so that you'd be able to disguise yourself," Gorodetsky told Chemistry World. "There's really not much out there in terms of inexpensive, biodegradable non-toxic materials that can be changed on the fly."

"We used reflectin, a protein that is important for cephalopod structural coloration, as a functional optical material," Gorodetsky told Nanowerk.

"We fabricated thin films from this protein, whose reflectance—and coloration—could be dynamically tuned over a range of over 600 nm and even into the infrared (in the presence of an appropriate stimulus). Our approach is environmentally friendly and compatible with a wide range of surfaces, potentially allowing many simple objects to acquire camouflage capabilities."

Infrared visualization equipment has a standard imaging range of 700 nm to 1200 nm, which pretty much matches the infrared region of the electromagnetic spectrum. Since this spectrum is not usually accessible to biologically derived materials, the team turned to the tunable optical properties of reflectin.

Mimicking Squid Skin

Seeking a way to mimic cephalopod skin, the researchers first developed a way to produce histidine-tagged reflectin A1 (RfA1). After experimenting with several substrates and surface treatments, the team found the most reliable formation of RfA1 thin films was achieved by spincasting 5 to 10 nm films of graphene oxide on a glass substrate and then spreading the RfA1 onto it.

The graphene oxide represents dark pigments found under layers of a cephalopod's skin, which contrasts with the color-changing cells.

Alon Gorodetsky is leading a team of reserachers to create camouflage coatings that change based on outside stimuli.

Depending on the thickness, the team reported distinct coloration, such as blue on 125 nm-thick film and orange on 207 nm-thick film. However, the researchers would have to find a way to significantly increase the thickness of their films in order to fall within the spectrum that cephalopods acheive.

"Given that some squid can dynamically modulate their skin reflectance across the entire visible spectrum and even out to near infrared wavelengths of ~800 nm, we postulated that it should also be possible to tune the reflectance of our RfA1 thin films across a similar, or even larger, wavelength range," Gorodetsky told Nanowerk.

The researchers used a variety of chemical stimuli to document the response of their RfA1 coatings. They found that a reversible shift in the reflectance spectra could be achieved by exposing the films to vapor from a concentrated acetic acid solution.

The team's future research will seek out a milder way to trigger the color change in their material so that coated materials could self-reconfigure in response to an external signal.

"You wouldn't necessarily want to use acetic acid—it's effectively dousing yourself with very concentrated vinegar! We'd like to find another stimulus—perhaps something mechanical or electrical—to induce the same change in coloration," Gorodetsky told Chemistry World.